Resolve native header values with @Header

The @Header annotation in spring-messaging now resolves values from the
nested "nativeHeaders" map as well as top-level header map values.

In case of ambiguity (a value that exists in both), the top-level map
value is used and a warning message is printed. This is unlikly in most
cases but can be resolved by prefixing the header value with
"nativeHeadres.myHeader".

Issue: SPR-11326